The present invention relates to a process of heat transfer printing the surface of shaped articles of polyacetals with sublimable dyes.
Heat transfer printing is a process for transferring sublimable disperse dyes to a receiving substrate by sublimation of the dye from a printed intermediate or auxiliary carrier and diffusion of the gaseous dye into the surface of the receiving substrate. The intermediate carrier may be of any of several appropriate materials including paper, metal, such as aluminum or steel, plastic, or fabrics optionally coated with various resins such as vinyl, polyurethane, polytetrafluoroethylene, or the like. All paper printing techniques, including gravure, lithography, rotary screen, and flexography, with their respective advantages and limitations have been used to print the carrier with the ink containing the sublimable disperse dyes. The dye is transferred by pressing the printed surface of the intermediate carrier into intimate contact with the substrate surface to be printed and applying sufficient heat and pressure to sublime the dye and cause diffusion of the dye into the substrate. Upon cooling, the dye condenses and is permanently adhered to the substrate.
It is commercial practice to print textile materials by sublimation printing from carriers printed or coated with disperse dyes in the form or an ink or paste. In the heat transfer printing of textiles, a wide variety of receiving substrates have been used including such synthetic polymers as polyacrylonitrile, polymers of vinyl compounds such as acrylic esters, acrylic amides, vinyl pyridine, vinyl chloride, vinylidene chloride, etc.; copolymers of dicyanoethylene and vinyl acetate; polyurethane; polyolefins; cellulose acetates; polyamides; and aromatic polyesters such as those from terephthalic acid and ethylene glycol or 1,4-di(hydroxymethyl)cyclohexane.
With the wide spread use of plastic articles in business and in the home, there is an obvious need for suitably decorating these articles. Such articles include tables, counter-tops, wall paneling, furniture, and the like. One particular important commercial application in business has been the sublimation printing or marking of business-machine keyboards. The sublimation dying or printing of plastic materials, however, has not been as readily accomplished as the heat transfer printing of textile materials. Among the problems which have been found is that the plastics do not readily absorb the dye, the heat required to sublime the dyes also is sufficient to soften the plastic receptor surface, as well as the dye adhesives on the transfer paper, and the paper and plastic fuse together.
U.S. Pat. No. 4,465,728 discloses a heat transfer process which attempts to alleviate some of these problems for heat transfer printing thermoplastics which will absorb sublimable dyes including polycarbonates, vinyls, acrylics, polystyrene, ABS (acrylonitrile-butadiene-styrene) and like extrudable polymers.
U.S. Pat. No. 4,406,662 discloses a process for improving the heat transfer printing of articles molded from filled methyl methacrylate homopolymers and copolymers of methyl methacrylate with other ethylenically unsaturated compounds (e.g., vinyl acetate, styrene, alkylacrylates, acrylonitrile, alkylmethacrylates, multifunctional acrylic monomers such as alkylene dimethacrylates and alkylene diacrylates) by preheating the articles.
While not directed specifically to heat transfer printing, U.S. Pat. No. 3,536,673 is directed to improving the dyeing of synthetic textiles, more particularly polyolefin textile fibers with disperse or metallized dye stuffs. In this particular patent, polyolefin textile fibers, in particular, polypropylene fibers are modified to enhance the dye-receptivity thereof by mixing the polyolefin with 5-25% by weight of polyoxymethylene.
Commercially, only a few thermoplastic resins have been printed by the heat transfer technique. These are polybutylene terephthalate such as marketed under the tradename Celanex from Celanese Corporatio as well as polycarbonate and polybutylene terephthalate and polycarbonate blends. A discussion of hot stamping plastic materials is given in "Hot Stamping Shows a World of Versatility", George Smoluk, Modern Plastics, December 1985, page 52.
One particular engineering thermoplastic which has not previously been printed satisfactorily by the heat transfer process is polyacetal or oxymethylene polymer resin. Oxymethylene polymers, having recurring --CH.sub.2 O-- units have been known for many years. They may be prepared by the polymerization of anhydrous formaldehyde or by the polymerization of trioxane which is a cyclic trimer of formaldehyde. The oxymethylene polymers are highly crystalline in nature and thus when molded or otherwise shaped into articles do not allow sufficiently deep penetration of the printing inks. Consequently, oxymethylene polymers have not been acceptable materials as substrates for the sublimation printing process.
It is widely known to modify the properties of oxymethylene polymers by incorporating additives therein. Thus, use of various additives have been suggested for improving the mechanical and physical properties such as the impact strength of oxymethylene polymers. The following patents are illustrative of such art.
U.S. Pat. No. 3,281,499 discloees improving the thermal stability and melting range of polyacetal polymers by reacting a polyacetal prepolymer having polymeric chains consisting essentially of a major proportion of recurring oxymethylene units and a monoethylenically unsaturated compound in the presence of a free radical yielding compound.
U.S. Pat. No. 3,476,832 discloses an oxymethylene polymer composition with improved impact resistance and superior flex resistance comprising a blend of a solid oxymethylene polymer and a rubbery polymeric material such as derived from diolefin-nitrile copolymers, lower alkyl acrylate homopolymers, and copolymers of lower alkyl acrylates with butadiene, ethylene, vinylethers, acrylonitrile, and the like.
U.S. Pat. No. 3,526,680 discloses improving the impact strength of oxymethylene polymers by blending therewith a copolymer of alpha-olefins and unsaturated carboxylic acids, which copolymer contains ions of mono to trivalent metals. Thus, copolymers of ethylene and acrylic acid or methacrylic acid which contain alkali metal ions can be used.
U.S. Pat. No. 3,704,275 modifies the properties of polyoxymethylenes by incorporating therein an inorganic nucleating agent and a polymer which is dispersed in the molding composition in the form of particles having a diameter in the range of from 0.1 to 5 microns. Various modifying polymers are disclosed including numerous olefinically unsaturated compounds including alpha-olefins such as polyethylene and ethylene/acrylic acid copolymers among numerous others.
U.S. Pat. No. 3,850,873 discloses glass fiber-reinforced polyoxymethylenes exhibiting improved mechanical properties by blending therewith a high molecular weight polyurethane. Polyurethane-modified oxymethylene polymer compositions are known to increase the impact strength relative to oxymethylene compositions without the polyurethane addition.
U.S. Pat. No. 4,201,849 discloses acetal resin compositions which exhibit very small mold shrinkage and mold warping by blending the acetal resin with a petroleum resin which has been prepared by polymerizing a cracked petroleum fraction boiling between -15.degree. and 200.degree. C. and containing unsaturated hydrocarbons.
U.S. Pat. No. 4,277,577 discloses an oxymethylene polymer molding composition of improved impact strength comprising a blend of an oxymethylene polymer, an elastomer, and a segmented thermoplastic copolyester or a polyurethane.
U.S. Pat. No. 4,424,307 discloses an oxymethylene polymer which is modified with 1,2-polybutadiene to enhance the physical properties, including tensile impact, elongation and work to break of the oxymethylene polymer.
Accordingly, while it has been suggested to modify oxymethylene polymers by adding thereto various polymeric materials, up until the present time, it has not been recognized that polymer additions to oxymethylene polymers can yield thermoplastic molding compositions which can be satisfactorily printed by the heat transfer printing process from otherwise substantially unprintable highly crystalline oxymethylene polymer.
Inasmuch as oxymethylene polymers have desirable physical and mechanical properties including good impact strength it would be advantageous to print molded articles formed therefrom and increase the use of this plastic in the home and in the business environment. Accordingly, a principle object of the present invention is to improve the printability of oxymethylene polymers by the sublimation printing process. Another object of the invention is to extend the use of polyacetal resins in various home and business environments where decorated engineering plastics have found use.
These and other objects, as well as the scope, nature, and utilization of the present invention, will be apparent to those skilled in the art from the following description and appended claims.